This invention relates generally to a miniature variable capacitor and manufacturing method thereof and more particularly to a miniature variable capacitor of a construction wherein the plates are coated with a film of a dielectric material, and, moreover, the spacing between the plates is made small thereby to afford simultaneously miniaturization and large maximum capacitance value. The invention relates also to method for manufacturing this capacitor.
In general, for providing a variable capacitor of large maximum capacitance, the following four approaches are considered.
a. Decreasing the gaps or clearances between the rotor plates and the stator plates. PA1 b. Interposing insulating material of high specific inductive capacity between the rotor plates and the stator plates. PA1 c. Increasing the number of rotor plates and stator plates. PA1 d. Increasing the mutually facing area of the rotor and stator plates.
By the above approaches (c) and (d) of increasing the number of plates and the mutually facing area of the plates, the variable capacitor is increased in size. Accordingly, these approaches cannot be resorted to when a miniaturization of a variable capacitor is desired. Therefore, in the case where a variable capacitor of large maximum capacitance and, moreover, miniature size is to be designed, the above approaches (a) and (b) are effective.
In resorting to the above approach (a), however, if the spacing between the rotor and stator plates were to be merely made smaller, there would be the possibility of contacting and short-circuiting between the rotor and stator plates and of failure of the capacitor to be formed because of unavoidable errors in dimensional precision, and there is also a limit to decreasing the gaps between the plates.
In one example of a known variable capacitor based on the practice of the above approach (b), sheets of dielectric material which are separate structures from the rotor and stator plates are interposed therebetween. In the assembly of this variable capacitor, however, it is necessary to transfix plate-by-plate the rotor plates on the rotor shaft and the stator plates on the stator support posts in stacked arrangement with spacers interposed between adjacent plates and, at the same time, to transfix the dielectric sheets similarly in interposed position between the plates in laminated arrangement. Consequently, the work of this assembling is extremely troublesome and gives rise to unavoidable inefficiency. Another problem is that this construction requires a large number of parts. Still another difficulty is that, particularly in the case where the dielectric sheets are made thin, and the spacing between the plates is made small, the dielectric sheets are pulled by frictional force arising between these sheets and the rotor plates when the rotor is rotated, and, in extreme cases wherein the rotor is rotated a large number of times, the dielectric sheets are damaged or broken and give rise to malfunctioning or breakdowns.
In another example of a known variable capacitor, dielectric films are caused to adhere to the plates by causing previously prepared dielectric films (sheets) to be melt-bonded to the surfaces of either the rotor plates or the stator plates or both. However, in this variable capacitor, also, the work of melt-bonding the dielectric films on the large number of plates is a laborious operation, and the production cost is high. Partidularly when the dielectric film is thin, the above mentioned melt-bonding work is made further difficult, and the rate of faulty or defective parts becomes high.
Particularly in the case of miniaturization of a variable capacitor, because of factors such as errors in the dimensions of the plates, the rotor shaft, the stator support posts, and other parts and inaccuracies in assembly, a rotor plate rarely exists exactly midway and parallel to the adjacent stator plates. Rather, in most cases in ordinary practice, a plate on one side is displaced toward a plate on the other side and, moreover, is not parallel thereto. Consequently, the plate of one plate group contacts the plate of the other plate group with a certain angle, and, particularly in the case where the spacing between the plates is made small, there arise problems such as the inability of the rotor to rotate smoothly, tendency of the dielectric film to be broken, and tendency of the capacitance to vary with use.